1. Field of the Invention
The present invention relates to a noise reducer and more particularly, to a noise reducer for reducing noise included in a digital image signal.
2. Description of the Prior Art
Conventionally, satellite broadcasting of television signals of an NTSC system has been performed utilizing radio waves of an SHF band.
Furthermore, it has been proposed that a new high-definition television receiver of a band reduction system developed by the Science and Technical Research Laboratories in Japan Broadcasting Corporation also receives high-definition television signals by satellite broadcasting.
The television receiver of an NTSC system which receives television signals of an NTSC system by satellite broadcasting and the high-definition television receiver which receives high-definition television signals by satellite broadcasting are adapted such that the received analog broadcasting signal is converted into a digital image signal and is processed in a digital manner.
On the other hand, in the field of processing of a digital image signal, a noise reducer utilizing a digital filter has been proposed in order to reduce noise included in an image signal. The noise reducer detects a signal for indicating the level difference between frames of the image signal and provides non-linear filter processing to the image signal in response to the level difference signal. For example, the noise reducer with a structure shown in FIG. 1 is described in "Digital Signal Processing of Image" (published by the Daily Industrial News, May 25, 1981), pp. 115-118.
In FIG. 1, a digital image signal of m parallel bits is supplied to an image signal input terminal 1 sequentially for one sample. The image signal input terminal 1 is connectdd to one input terminal of an adder 2 and a negative input terminal of a subtracter 3. The digital image signal of m bits output from the adder 2 is applied to a frame memory 4 and an image signal output terminal 5. The frame memory 4 delays the input image signal by one or several frames, the output thereof being applied to a positive input terminal of the subtracter 3. Thus, the subtracter 3 subtracts the image sigaal supplied from the image signal input terminal 1 from the image signal supplied from the frame memory 4 and outputs the same. An output of the the subtracter 3 is applied to a read-only memory (referred to as a ROM hereinafter) 6. In the ROM 6, a value which is K (0.ltoreq.K&lt;1) times the level value of an output signal of the subtracter 3 is previousl written into each address designated by the level of the output signal of the subtracter 3. As described below, the parameter K is a non-linear parameter which changes in accordance with the level value of the output signal of the subtracter 3. Thus, the ROM 6 outputs a signal of m bits for reducing noise, which signal corresponds to K times of the level value of the output signal of the subtracter 3. The signal of m bits for reducing noise is applied to the other input terminal of the adder 2.
The above described adder 2, the subtracter 3, the frame memory 4 and the ROM 6 constitute a digital filter 7 of a recursive structure type.
Description is now made on operation of the circuit in FIG. 1.
The level difference between the image signal input from the image signal input terminal 1 and an image signal which was delayed by, for example, one frame, by the frame memory 4 (that is, an image signal one frame before) is determinated by the subtracter 3. Thus, the level difference signal between frames of the image signal is output from the subtracter 3 and is applied to the ROM 6. In the ROM 6, address is designated in accordance with the absolute value of the level of the level difference signal and a value which is K times the aboolute value is read out. In addition, in the ROM 6, a sign (+or -) of an input signal (level difference signal) is added to the value which is K times the absolute value and then, the result is output to the adder 2. In the adder 2, the image signal input from the image signal input terminal 1 is added to the output signal of the ROM 6 and a digital image signal having reduced noise is output. The digital image signal having reduced noise is output to the image signal output terminal 5.
The above described example utilizes the fact that the image signal shows a strong correlation between frames. For example, if the image signal supplied to the image signal input terminal 1 is a still picture signal, the image signal one frame before and the current image signal coincide with each other. Therefore, the output signal of the subtracter 3, i.e., the level difference signal is an extracted noise component superposed on the image signal (having inverted polarity). When the level difference signal is multiplied by K in the ROM 6 and the result is added to the image signal from the image signal input terminal 1 in the adder 2, an image signal having reduced noise by averaging noise with respect to time is obtaihed in the image signal output terminal 5.
As described in the foregoing, if the image signal input from the image signal input terminal 1 is a still picture signal, the level difference signal of the subtracter 3 is a signal including a noise component having inverted polarity. Therefore, the larger the parameter K is, the larger the effect for reducing noise is, so that the noise level of the image signal output from the image signal output terminal 5 becomes low.
On the other hand, if the image signal inputted from the image signal input terminal 1 is a moving picture signal, the correlation between frames becomes weak, so that the level of the level difference signal from the subtracter 3 largely changes due to movement of an image between frames. In this case, if the parameter K is set to a large constant value irrespective of the level of the level difference signal in order to increase the effect for reducing noise components, the level change of the image signal due to movement of the image is averaged with respect to time. Therefore, the image signal obtained from the image signal output terminal 5 becomes a signal having blurred profile of the moving picture portion, so that the picture quality is deteriorated.
Therefore, conventionally, the above described parameter K has been changed in accordance with a preset characteristic depending on the magnitude of the absolute value of the level of the level difference signal of the subtracter 3. More specifically, when the absolute value of the level of the level difference signal becomes large, it is considered that the image moves and hence, the parameter K is decreased. On the other hand, when the absolute value of the level of the level difference signal becomes small, it is considered that the change is caused by noise and thus the parameter K is increased. Then, the non-linear filter processing of the image signal input from the image signal input terminal 1 is made. As a result, the quality of the moving picture is prevented from deteriorating, so that noise can be reduced.
The change characteristic of the parameter K is set in trial and error manner, so that the value of the parameter K in the latter case is generally set to 0.5-0.8.
In a digital image processing apparatus such as a television receiver which receives satellite broadcasting, the noise reducer shown in FIG. 1 or almost the same noise reducer as the noise reducer (for example, a noise reducer comprising a combinational logic circuit substituted for the ROM 6 or a noise reducer comprising a ROM having smaller capacitance than that of the ROM 6 and the combination logic circuit) is provided in order to reduce noise of the digital image signal of the received broadcasting signal.
In a television receiver of an NTSC system which receives a television signal of an NTSC system by satellite broadcasting, the arrangement of picture elements is the same in each frame, so that the frame memory 4 is formed of a memory having storage capacitance of one frame (=two fields). On the other hand, in a high-definition television receiver which receives a high-definition television signal by satellite broadcasting, the arrangement of picture elements is the same only for every two frames, so that the frame memory 4 is formed of a memory having storage capacitance of two frames (=four fields).
When the noise reducer in FIG. 1 is provided in the television receiver, an image signal and the noise level largely change depending on the receiving state or the like, so that the following problems occur. More specifically, when the change characteristic of the parameter K is set to, for example, a characteristic of surely preventing a blur of a moving picture, that is, a characteristic in which the parameter K is decreased at a relative low level of the level difference signal, a value of the parameter K becomes small when the magnitude of noise is increased due to deterioration of the receiving state or the like, considering that the change is caused by movement of the image, so that noise is not liable to be satisfactorily reduced. On the other hand, when the change characteristic of the parameter K is set to a characteristic of surely reducing noise, that is, a characteristic in which the parameter K is increased to a relatively high level of the level difference signal, the moving picture is liable to be remarkably blurred. Thus, in the noise reducer in FIG. 1, it was difficult to set the change characteristic of the parameter K to the best characteristic in accordance with the receiving state or the like.
Particularly, in the television receiver which receives the television signal by satellite broadcasting, C/N (carrier power/noise) of a received signal changes depending on the weather condition. For example, since in rainy weather, C/N becomes lower than that in fine weather, the receiving state is deteriorated, so that it is remarkably difficult to set the change characteristic of te parameter K.
Furthermore, in satellite broadcasting of the high-definition television signal, the base bandwidth is larger than that of the conventional satellite broadcasting of an NTSC system, so that it is difficult to widen deviation of FM modulation of broadcasting signals. Thus, even if C/N of the received broadcasting signal is the same as that of the broadcasting signal of satellite broadcasting of an NTSC system, the magnitude of noise relative to a demodulated image signal is larger than that of a demodulated broadcasting signal of satellite broadcasting signal of satellite broadcasting of an NTSC system. Therefore, in the noise reducer provided in the high-definition television receiver which receives the high-definition television signal by satellite broadcasting, it is further difficult to set the change characteristic of the parameter K.
When the high-definition television signal by satellite broadcasting is received and processed, it is most important to make a reproduced image high quality, so that the noise reducer must satisfactorily reduce noise at the some sacrifice of the blur due to movement of the image.
On the other hand, Japanese Patent Laying-Open Gazette No. 37018/1980 or No. 193179/1982 describes a noise reducer in which a moving vector detecting circuit for detecting movement of an image and a moving vector correcting circuit for vector-correcting an image signal output, for example, from the frame memory 4 to the subtracter 3 by a detection signal of the moving vector detecting circuit are added to the structure of the noise reducer in FIG. 1. The noise reducer corrects deviation between the image signal output, for example, from the frame memory 4 to the subtracter 3 and the image signal input from the image signal input terminal 1 relative to the moving portion, so that effect of the moving portion to the level difference signal of the subtracter 3 may be decreased. Thus, the change characteristic of the parameter K is set to a characteristic in which a value of the parameter K becomes relatively large when the level of the level difference signal becomes large, so that the above described problem can be solved.
However, in the above described moving vector detecting circuit and the moving vector correcting circuit, the structures thereof are complicated and large-sized, and it is difiicult to surely make correction. Therefore, there occurs a new problem that when the moving vector detecting circuit and the moving vector correcting circuit are added, the noise reducer becomes complicated and large-sized. In addition, there is also a problem that when connection was not suitably made, noise can not be satisfactorily reduced.